Plants are irradiated with a light emission obtainable by energizing incandescent lamp (3) with ac or de voltage that exceeds its rating but does not exceed 150% thereof. Energization of such overvoltage allows incandescent lamp (3) to emit a light having a spectral distribution similar to that of the sublight. Irradiation of the light emission effectively and efficiently stimulates the germination and early growth of seedlings, propagation and early growth of cuttings, tissue culture propagation, and etc.